The recipe I'm using calls for 3/4 to 1 full box powdered sugar.
How much is in a full box?
We are unable to purchase boxes where I live... or at least to my knowledge.

A full measured cup of powdered sugar weighs 92 grams. Any recipe will call for measurements rather than weights as the degree of fineness varies with manufacturer. I use a my electric spice grinder, cleaning thoroughly after each use. You can use a food processor to make powdered sugar by pulsing to the desired fineness.